I'll be hiking through Baxter State Park, in Maine, in June. I am traveling as light as possible as I will be crossing multiple ridges. I have a 30F sleeping bag and a 0F bag. The 30F bag is much smaller. Can I get away with it?
According to Baxter State Park Monthly Weather Forecast, the record lows for May, June and July are 18 °F (-8°C), 28 °F (-2°C) and 32 F (0°C), respectively. The average lows for May, June and July are 39 F (4°C), 48 F (9°C) and 54 F (12°C), respectively.
Climate Baxter State Park says much the same thing.
As you know, the altitude varies significantly. The approximate elevation is 1,447 feet (441 m), and the highest point (Mt. Katahdin) is 5,267 feet (1605 m). Figure 3 to 5 degrees (F) temperature drop per 1,000 feet of elevation (6–7°C per 1000 metre) gain, but terrain and wind are always factors.
If you are camping at the lower elevations of the Park, in a spot sheltered from wind, your 30 °F (-1 °C) bag should be fine, but maybe not at the top of Katahdin.
Katahdin is the highest summit in the area and camping is not permitted. Surrounding mountains are not very high and you should be able to camp at the bottom of them with some minimal planning if you're concerned about the temperature.
A 30F bag is most likely what you will end up using and I would bring that with a few extra light layers that I can use during the day and at night if needed. e.g. a light puffy & windbreaker pants.
If you use lean-to's, these can something feel colder because they are fully open on one side. I have used a 40F quilt in Baxter State Park, in early July, without any trouble.